1. Field of the Invention
The present invention relates to multi-channel audio technology and, in particular, to multi-channel audio applications in connection with headphone technologies.
2. Description of the Related Art
The international patent applications WO 99/49574 and WO 99/14983 disclose audio signal processing technologies for driving a pair of oppositely arranged headphone loudspeakers in order for a user to get a spatial perception of the audio scene via the two headphones, which is not only a stereo representation but a multi-channel representation. Thus, the listener will get, via his or her headphones, a spatial perception of an audio piece which in the best case equals his or her spatial perception, should the user be sitting in a reproduction room which is exemplarily equipped with a 5.1 audio system. For this purpose, for each headphone loudspeaker, each channel of the multi-channel audio piece or the multi-channel audio datastream, as is illustrated in FIG. 2, is supplied to a separate filter, whereupon the respective filtered channels belonging together are added, as will be illustrated subsequently.
On a left side in FIG. 2, there are the multi-channel inputs 20 which together represent a multi-channel representation of the audio piece or the audio datastream. Such a scenario is exemplarily schematically shown in FIG. 10. FIG. 10 shows a reproduction space 200 in which a so-called 5.1 audio system is arranged. The 5.1 audio system includes a center loudspeaker 201, a front-left loudspeaker 202, a front-right loudspeaker 203, a back-left loudspeaker 204 and a back-right loudspeaker 205. A 5.1 audio system comprises an additional subwoofer 206 which is also referred to as low-frequency enhancement channel. In the so-called “sweet spot” of the reproduction space 200, there is a listener 207 wearing a headphone 208 comprising a left headphone loudspeaker 209 and a right headphone loudspeaker 210.
The processing means shown in FIG. 2 is formed to filter each channel 1, 2, 3 of the multi-channel inputs 20 by a filter HiL describing the sound channel from the loudspeaker to the left loudspeaker 209 in FIG. 10 and to additionally filter the same channel by a filter HiR representing the sound from one of the five loudspeakers to the right ear or the right loudspeaker 210 of the headphone 208.
If, for example, channel 1 in FIG. 2 were the front-left channel emitted by the loudspeaker 202 in FIG. 10, the filter HiL would represent the channel indicated by a broken line 212, whereas the filter HiR would represent the channel indicated by a broken line 213. As is exemplarily indicated in FIG. 10 by a broken line 214, the left headphone loudspeaker 209 does not only receive the direct sound, but also early reflections at an edge of the reproduction space and, of course, also late reflections expressed in a diffuse reverberation.
Such a filter representation is illustrated in FIG. 11. In particular, FIG. 11 shows a schematic example of an impulse response of a filter, such as, for example, of the filter HiL of FIG. 2. The direct or primary sound illustrated in FIG. 11 by the line 212 is represented by a peak at the beginning of the filter, whereas early reflections, as are illustrated exemplarily in FIG. 10 by 214, are reproduced by a center region having several (discrete) small peaks in FIG. 11. The diffuse reverberation is typically no longer resolved for individual peaks, since the sound of the loudspeaker 202 in principle is reflected arbitrarily frequently, wherein the energy of course decreases with each reflection and additional propagation distance, as is illustrated by the decreasing energy in the back portion which in FIG. 11 is referred to as “diffuse reverberation”.
Each filter shown in FIG. 2 thus includes a filter impulse response roughly having a profile as is shown by the schematic impulse response illustration of FIG. 11. It is obvious that the individual filter impulse response will depend on the reproduction space, the positioning of the loudspeakers, possible attenuation features in the reproduction space, for example due to several persons present or due to furniture in the reproduction space, and ideally also on the characteristics of the individual loudspeakers 201 to 206.
The fact that the signals of all loudspeakers are superposed at the ear of the listener 207 is illustrated by the adders 22 and 23 in FIG. 2. Thus, each channel is filtered by a corresponding filter for the left ear to then simply add up the signals output by the filters which are destined for the left ear to obtain the headphone output signal for the left ear L. In analogy, an addition by the adder 23 for the right ear or the right headphone loudspeaker 210 in FIG. 10 is performed to obtain the headphone output signal for the right ear by superposing all the loudspeaker signals filtered by a corresponding filter for the right ear.
Due to the fact that, apart from the direct sound, there are also early reflections and, in particular, a diffuse reverberation, which is of particularly high importance for the space perception, in order for the tone not to sound synthetic or “awkward” but to give the listener the impression that he or she is actually sitting in a concert room with its acoustic characteristics, impulse responses of the individual filters 21 will all be of considerable lengths. The convolution of each individual multi-channel of the multi-channel representation having two filters already results in a considerable computing task. Since two filters are necessary for each individual multi-channel, namely one for the left ear and another one for the right ear, when the subwoofer channel is also treated separately, a total amount of 12 completely different filters is necessary for a headphone reproduction of a 5.1 multi-channel representation. All filters have, as becomes obvious from FIG. 11, a very long impulse response to be able to not only consider the direct sound but also early reflections and the diffuse reverberation, which really only gives an audio piece the proper sound reproduction and a good spatial impression.
In order to put the well-known concept into practice, apart from a multi-channel player 220, as is shown in FIG. 10, very complicated virtual sound processing 222 is necessary, which provides the signals for the two loudspeakers 209 and 210 represented by lines 224 and 226 in FIG. 10.
Headphone systems for generating a multi-channel headphone sound are complicated, bulky and expensive, which is due to the high computing power, the high current requirement for the high computing power necessary and the high working memory requirements for the evaluations to be performed of the impulse response and the high volume or expensive elements for the player connected thereto. Applications of this kind are thus tied to home PC sound cards or laptop sound cards or home stereo systems.
In particular, the multi-channel headphone sound remains inaccessible for the continually increasing market of mobile players, such as, for example, mobile CD players, or, in particular, hardware players, since the calculating requirements for filtering the multi-channels with exemplarily 12 different filters cannot be realized in this price segment neither with regard to the processor resources nor with regard to the current requirements of typically battery-driven apparatuses. This refers to a price segment at the bottom (lower) end of the scale. However, this very price segment is economically very interesting due to the high numbers of pieces.